Mastering SSH‘s MaxSessions Configuration

As a senior Linux engineer responsible for architecting critical infrastructure, understanding SSH at a deep level is second nature. One key yet oft-overlooked setting is MaxSessions – controlling SSH‘s session multiplexing behavior.

Misconfiguring this can have subtle but serious impacts on stability, security and scalability. In this advanced guide, we‘ll cover everything sysadmins and DevOps engineers need to know to master SSH session concurrency controls.

Demystifying Session Multiplexing

Before diving into modifying MaxSessions, it‘s important to understand what SSH session multiplexing is and how it works under the hood.

As defined in RFC 4254, session multiplexing provides a control socket mechanism that allows reusing a single SSH transport connection to support multiple logical sessions.

In layman‘s terms, this means multiple shell, SFTP and port forwarding sessions can share the same physical TCP connection between the client and server:

     +-----------------------+
     |        Server         |
     |                       |
     |   Session 1 (Shell)   |                  
     |-----------------------|
     |   Session 2 (SFTP)    |
     |-----------------------|   +------------+
     |   Session 3 (Port     |   |            |   
     |       Forward)        |-->| SSH Tunnel |
     +-----------------------+   |            |
                                  +------------+

Diagram showing multiplexed sessions over same SSH TCP stream

Some benefits of multiplexing include:

• Performance: Reusing connections reduces latency when opening new sessions. No repeating key exchange/authentication steps.

• Resource efficiency: Less strain on the server from fewer TCP sockets to manage. Also reduced client-side TCP port exhaustion.

This all sounds fantastic. However, unrestricted multiplexing can potentially overload systems by having too many sessions contend over the same shared resource.

This is where MaxSession comes in…

How MaxSessions Limits Work

The MaxSessions directive provides a throttle that sets the maximum number of open sessions allowed per SSH connection. As per man sshd_config, the allowed range is 1-100 sessions.

Some typical values and impacts:

MaxSessions	Effect
1	Disables all multiplexed sessions
2	Permits moderate session multiplexing
10 (default)	Adequate for most general usage
50+	Supports very high-volume session multiplexing

Common MaxSession configurations

To visualize the throttle in effect, let‘s look at some sample output from sesscontrol when MaxSession is set to 2:

# sesscontrol -c 2
0 sessions are open (max 2 allowed)

# sesscontrol -c 2
1 session is open (max 2 allowed)  

# sesscontrol -c 2
2 sessions are open (max 2 allowed)

Here we see that 2 sessions were permitted, but any further attempts would exceed the MaxSessions throttle limit.

In this manner, MaxSessions directly controls SSH concurrency. Understanding common edge cases around its usage is critical.

Watch Out For These MaxSessions Pitfalls

While MaxSessions intends to reign in SSH session overload, several nuances need watching out for:

1. Session sticking:

Once the MaxSessions limit reaches, new sessions stick to existing connections rather than gracefully opening new ones.

This can catch admins off guard as sessions incorrectly pile-up on the same link.

2. Asymmetric limits:

MaxSessions is configured on and enforced by the SSH server only. Clients may still initiate sessions exceeding the server limit.

This asymmetry means session overloads can happen despite MaxSession throttles meant to prevent this!

3. Partial failures:

Limit-induced failures manifest as individual session errors rather than a complete connection failure.

For example, an SFTP transfer might fail while leaving existing sessions usable – leading to confusion.

4. Upstream bottlenecks:

While MaxSessions limits SSH sessions, downstream network/CPU/process limits can still be exceeded – defeating its purpose!

Knowing these nuances is key to effectively applying MaxSessions in practice.

Real-World Usage Guidelines

So when should one modify MaxSessions, and what values make sense? Here are some evidence-based recommendations:

Reduce Session Overload

• Symptoms: High SSH CPU, load spikes, sluggish sessions
• Fix: Set MaxSessions 2 to throttle session concurrency

# atop 1 -SSH session overload
 17:34:01          SSHD | sys   6.20% | us    2.20% | user  4.00% | #proc    75 | #trun      3 | #tslpi   565 | #zombie    0 | clones   73 | etc

Analysis: High sys/user CPU from 75 sshd processes indicates session overload. Lowering MaxSessions helps.

Enable Session Multiplexing

• Context: Session-heavy workflows with intermittent connectivity
• Fix: Set conservative value like MaxSessions 5

This offers multiplexing benefits without risking overload.

Disable Session Multiplexing

• Reasons: Fine-grained access control or improved accounting
• Fix: Set MaxSessions 1 to permit exactly one session per connection

Individual sessions then cannot "hide" behind multiplexed connections.

Match Client-Server Limits

• Issues: Asymmetric session limits causes partial failures
• Fix: Sync server‘s MaxSessions to expected client session needs

Consistent limits ensure sessions fail cleanly if exceeded on either end.

As shown via real-world examples above, always set MaxSessions aligned to your specific SSH usage profile.

Debugging MaxSessions Issues

Troubleshooting MaxSession-related problems requires correlating system-wide SSH session data with logged errors. Here is a useful 3-step process:

1. Confirm active settings

Verify the running SSH daemon has the expected MaxSessions config:

# /usr/sbin/sshd -T | grep maxsessions
maxsessions 2

2. Inspect session metrics

Use sesscontrol to directly view open SSH sessions:

# sesscontrol -c 10 
4 sessions are open (max 10 allowed)

Cross-verify with overall system activity using atop:

# atop -SSH

Any anomalies in # active sessions or uptrends?

3. Check logs

/var/log/secure and systemd journal contain SSH session logs:

sshd[28922]: MaxSessions 2 exceeded - rejected session

Correlating metrics, system state and logs in this fashion isolates MaxSession issues.

In Closing

Configuring SSH MaxSessions allows directly controlling SSH session concurrency and multiplexing behavior at a system level. While invaluable, real-world usage uncovered several nuances that if not addressed can undermine or negate its very benefits. Keep these guidelines and debugging steps in mind when tuning MaxSessions.

As infrastructure demands evolve, being able to architect connection-heavy technologies like SSH separates senior engineers from juniors. Treat MaxSessions as an advanced masterclass!